Method of manufacturing interconnection structural body

ABSTRACT

Disclosed is a method for producing a circuit structure having an insulator layer comprising a porous silicon oxide thin film, which comprises (1) forming a preliminary insulator layer comprising a silicon oxide-organic polymer composite thin film formed on a substrate, which silicon oxide-organic polymer composite thin film comprises a silicon oxide having an organic polymer dispersed therein, (2) forming, in the preliminary insulator layer, a groove which defines a pattern for a circuit, (3) forming, in the groove, a metal layer which functions as a circuit, and (4) removing the organic polymer from the preliminary insulator layer to render the preliminary insulator layer porous, thereby converting the preliminary insulator layer to an insulator layer comprising a porous silicon oxide thin film. By the method of the present invention, the capacitance between mutually adjacent circuit lines (line-to-line capacitance) in the circuit structure can be lowered.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a circuitstructure. More particularly, the present invention is concerned with amethod for producing a circuit structure having an insulator layercomprising a porous silicon oxide thin film, which comprises (1) forminga preliminary insulator layer comprising a silicon oxide-organic polymercomposite thin film formed on a substrate, which silicon oxide-organicpolymer composite thin film comprises a silicon oxide having an organicpolymer dispersed therein, (2) forming, in the preliminary insulatorlayer, a groove which defines a pattern for a circuit, (3) forming, inthe groove, a metal layer which functions as a circuit, and (4) removingthe organic polymer from the preliminary insulator layer to render thepreliminary insulator layer porous, thereby converting the preliminaryinsulator layer to an insulator layer comprising a porous silicon oxidethin film. By the method of the present invention, not only can theline-to-line capacitance in the circuit structure be lowered, but also alow resistance metal, such as copper or silver, can be used as amaterial for a circuit, so that it has become possible to produce anexcellent circuit structure in which the delay in the transmission ofthe electric signal (this phenomenon is the so-called “interconnectdelay”) is greatly suppressed, as compared to the case of theconventional circuit structures. Further, by the method of the presentinvention, it has become possible to produce such an excellent circuitstructure with high efficiency.

The present invention is also concerned with a multilayer circuit boardcomprising the above-mentioned excellent circuit structure, and asemiconductor device comprising the above-mentioned excellent circuitstructure.

2. Prior Art

Conventionally, as a material for an insulator layer used in amultilayer circuit of a semiconductor device, such as an LSI, anon-porous silicon oxide or a silicon oxide having incorporated thereina fluorine atom or an organic group has been used. However, thedielectric constants of such materials are high. In recent years, thedensity of the circuit in a semiconductor device, such as an LSI, hasbeen increasing and, hence, the distance between the adjacent conductivepaths of the circuit has been decreasing. Due to the decreased distancebetween the adjacent conductive paths of the circuit, the adjacentconductive paths function as a capacitor. In this case, when thedielectric constant of the insulator layer is high, a problem arisesthat the capacitance in the circuit becomes large, thereby leading tothe delay in the transmittance of the electric signal through thecircuit (i.e., interconnect delay). Therefore, in order to lower thedielectric constant of the insulator layer, it is attempted to use aninsulator layer composed of a composite of a silicon oxide and anorganic polymer, or a porous silicon oxide, i.e., a composite of asilicon oxide and air which has a dielectric constant of approximately1.

In the future, when the density of the circuit of a semiconductordevice, such as an LSI, has further increased, the importance of thealleviation of the interconnect delay would become much greater thanthat at present. Therefore, in addition to the lowering of thedielectric constant of the insulator layer, it also becomes necessary touse, as a material for a circuit, a low resistance metal represented bycopper and silver instead of the conventionally used aluminum. However,in the conventional process for producing a circuit structure, it isdifficult to use such a low resistance metal as a material for acircuit. The reason is as follows. The conventional process comprises:forming a metal layer on the entire surface of a substrate; forming, onthe metal layer, a photoresist pattern (protective layer), whichcorresponds to the desired circuit pattern; removing the non-protectedportions of the metal layer (i.e., the portions of the metal layer whichare not covered by the photoresist pattern) by the conventional etchingprocess, thereby forming a circuit on the substrate; and coating thecircuit with an insulator layer. The above-mentioned conventionaletching process utilizes a substance which is capable of forming a highvapor pressure compound with the metal used for forming the circuit. Insuch a conventional etching process, the protected portions of the metallayer, which are covered by the photoresist, are not eroded, and onlythe non-protected portions of the metal layer are converted into a highvapor pressure substance, so that the non-protected portions of themetal layer are selectively removed. However, when the metal layer isformed from a low resistance metal represented by copper and silver,such a low resistance metal cannot form a high vapor pressure compound,but forms only a low vapor pressure compound, so that the circuit cannotbe formed by the conventional etching process. Therefore, in theconventional techniques, the above-mentioned low resistance metal cannotbe used as a material for forming a circuit.

In order to solve the above-mentioned problems, the so-called “damasceneprocess” has been proposed. The damascene process comprises: forming aninsulator layer on a substrate; forming, in the insulator layer, agroove which defines a pattern for a circuit; forming a layer of a metalon the entire surface of the insulator layer, so that the groove iscompletely filled with the metal; removing the metal which is notpresent in the groove by etch back method utilizing a plasma or chemicalmechanical polishing (CMP) method, so that the surfaces of the insulatorlayer and the surface of the metal layer (which functions as a circuit)are exposed (with respect to the damascene method, reference can be madeto, for example, “International Electron Device Meeting TechnicalDigest” (1997), p. 773-776, and Unexamined Japanese Patent ApplicationLaid-Open Specification No. 62-102543). Thus, in the damascene process,the formation of the circuit need not be conducted by the conventionaletching method, but can be conducted by etch back method utilizing aplasma or chemical mechanical polishing (CMP) method. Therefore, in thedamascene process, a low resistance metal, such as copper or silver, canbe used as a material for the circuit.

Further, as well known in the art, when the damascene process isemployed for the production of a multilayer circuit board comprising alaminate of a plurality of circuit structures, the number of stepsrequired for the production is small, as compared to that in theconventional process. Therefore, the damascene process is veryadvantageous for reducing the production cost.

Specifically, in the production of a multilayer circuit board by aconventional method, the formation of a new (upper) circuit structure ona (lower) circuit structure which has been already formed is conductedby a process comprising the steps of: forming, on the lower circuitstructure, an insulator layer for separating the lower circuit structurefrom the upper circuit structure to be formed; forming, in the insulatorlayer, a vertical through-hole for accommodating therein a verticalconductive path which electrically connects the lower circuit structureand the upper circuit structure to be formed; forming a verticalconductive path in the through-hole; and forming the upper circuitstructure in the same manner as mentioned above in connection with theconventional process for producing a circuit structure.

By contrast, in the damascene process, after an insulator layer forseparating the lower circuit structure from the upper circuit structure(to be formed) is formed on the lower circuit structure, the formationof the vertical through-hole for accommodating therein a verticalconductive path (which electrically connects the lower circuit structureand the upper circuit structure to be formed) and the formation (in theinsulator layer separating the lower circuit structure from the uppercircuit structure) of the groove which defines a pattern for a circuitof the upper circuit structure can be conducted in a single step. Then,the above-mentioned vertical through-hole and the above-mentioned groovecan be simultaneously filled with the metal. After the verticalthrough-hole and the groove are filled with the metal, the upper circuitstructure can be completed by only removing the metal which is notpresent in the groove by the above-mentioned etch back method utilizinga plasma or the above-mentioned chemical mechanical polishing (CMP)method. Thus, the damascene process is very advantageous not only inthat a low resistance metal, such as copper or silver, can be used, butalso in that the number of steps required for the production of amultilayer circuit board is small, as compared to that in theconventional process.

However, the conventional damascene process has the following problem.The insulator layer used in the conventional damascene process iscomposed of a silicon oxide and is produced by plasma chemical vapordeposition (CVD). This silicon oxide insulator layer has a highdielectric constant and, hence, the interconnect delay cannot besatisfactorily suppressed. In order to solve this problem, it has beenproposed to employ an insulator layer having a dielectric constant lowerthan that of the above-mentioned silicon oxide insulator layer producedby plasma CVD.

For example, it is known to use an insulator layer composed of acomposite of a silicon oxide and an organic polymer. With respect to theabove-mentioned organic polymer used in such an insulator layer, theorganic polymer needs to have a low dielectric constant so as to obtainan insulator layer having a satisfactorily low dielectric constant.Examples of such organic polymers include paraquinoxaline (dielectricconstant: 2.70) reported by Hedrick et al (Polymer, Vol. 34, p. 4717(1993)) and polyquinoline (dielectric constant: 2.5) reported by Monk etal (Polymers for Dielectric and Photonic Applications, p. 119, (1993)).

However, the above-mentioned silicon oxide-organic polymer compositecannot be used for producing an insulator layer having a satisfactorilylow dielectric constant.

In this situation, a technique employing a porous silicon oxide film asthe insulator layer has been drawing attention. In this technique, thedielectric constant of a silicon oxide film is lowered by rendering thesilicon oxide film porous to thereby obtain a composite of the siliconoxide and air. This technique is described in, for example, U.S. Pat.No. 5,472,913. Specifically, with respect to the insulator layer, thispatent document describes the use of a porous silicon oxide filmproduced by a method comprising: subjecting a tetralkoxysilane tohydrolysis and dehydration condensation in an alcohol to form a wetsilicon oxide gel film; and immersing the formed wet silicon oxide gelfilm in a solution of trimethylchlorosilane (which is a silylationagent) to thereby render water repellent the surface of the wet siliconoxide gel film, followed by drying under atmospheric pressure. However,it has conventionally been very difficult to produce a circuit structurehaving a insulator layer composed of a porous silicon oxide by thedamascene process, for the following reason.

In the above-mentioned U.S. Pat. No. 5,472,913, the porous silicon oxidefilm obtained by the above-mentioned method is covered with a protectivelayer composed of a non-porous silicon oxide. Then, the protective layeris etched by lithography method, followed by the formation of theabove-mentioned vertical through-hole by etching the porous siliconoxide film (insulator layer) under conditions different from thoseemployed in the etching of the protective layer. However, even when thethickness of the protective layer composed of the non-porous siliconoxide is only slightly uneven, the porous silicon oxide (positionedbelow the protective layer) is caused to be unevenly etched at a speedseveral times higher than that in the case of the etching of theprotective layer. Therefore, the thickness of the protective layer andthe etching conditions must be strictly controlled, thereby causing agreat difficulty.

Thus, generally, when it is attempted to process finely the poroussilicon oxide film so as to form a circuit on the porous silicon oxidefilm, a great difficulty is encountered due to the poor resistance ofthe porous silicon oxide against the dry etching and the like.

Further, the damascene process has the following problem. In thedamascene process, a metal layer is formed on the insulator layer havinga fine groove which defines the pattern for a circuit. Therefore, whenthe porous silicon oxide film is used as the insulator layer, there is adanger that the metal intrudes into the pores of the insulator layerduring the formation of the metal layer. This problem also rendersdifficult the use of the porous silicon oxide film in the damasceneprocess.

Further, in the damascene process, it is necessary to employ an etchback method or a chemical mechanical polishing (CMP) method for removingthe metal which is not present in the groove or removing a part of theinsulator layer (for forming a vertical through-hole). However, when theporous silicon oxide film is used as the insulator layer, disadvantagesare likely to be caused during the etch back or the CMP. Specifically,in the case of the etch back method utilizing a plasma, when the poroussilicon oxide is exposed to the plasma, the gas generated during theetching is trapped in the pores of the insulator layer, or the insulatorlayer is damaged. Further, in the case of the CMP, since an acidic oralkaline aqueous slurry containing abrasive particles is used, there isa danger that the insulator layer is dissolved or damaged.

In order to solve the above-mentioned problems, Zielinski et al proposea method in which, prior to the formation of the metal layer and thesubsequent CMP, the porous silicon oxide film having a groove (whichdefines the pattern for a circuit) is covered with a protective layercomposed of a non-porous silicon oxide film (International ElectronDevice Meeting Technical Digest (1997) p. 936-938). In this method, theabrasion of the porous silicon oxide film can be prevented by theprotective layer. Therefore, this method is free from theabove-mentioned problem of the damage to the porous silicon oxide film.Further, since the side walls of the groove are also protected by thenonporous silicon oxide film, the danger of the intrusion of the metalinto the pores of the insulator layer can be alleviated.

However, this method has a problem that the step of formation of theprotective layer is necessary, so that the process for forming thecircuit structure becomes cumbersome. In addition, this method has aproblem that the non-porous silicon oxide film (having a high dielectricconstant) remains on a part of the surface of the insulator layer andthe side walls of the groove, so that, despite that the porous siliconoxide film is used as the insulator layer, a satisfactorily lowdielectric constant cannot be achieved.

SUMMARY OF THE INVENTION

In this situation, the present inventors have made extensive andintensive studies toward solving the above-mentioned problemsaccompanying the prior art and developing a method for efficientlyproducing a circuit structure having an insulator layer comprising aporous silicon oxide film which has a high dielectric constant by thecommercially advantageous damascene process. As a result, it hasunexpectedly been found that this object can be attained by a methodcomprising: (1) forming a preliminary insulator layer comprising asilicon oxide-organic polymer composite thin film formed on a substrate,which silicon oxide-organic polymer composite thin film comprises asilicon oxide having an organic polymer dispersed therein, (2) forming,in the preliminary insulator layer, a groove which defines a pattern fora circuit, (3) forming, in the groove, a metal layer which functions asa circuit, and (4) removing the organic polymer from the preliminaryinsulator layer to render the preliminary insulator layer porous,thereby converting the preliminary insulator layer to an insulator layercomprising a porous silicon oxide thin film. The present invention hasbeen completed, based on this novel finding.

Accordingly, it is a primary object of the present invention to providea method based on the damascene process, which can be used for easilyand efficiently producing a circuit structure in which the capacitancebetween the adjacent conductive paths of the circuit (i.e., line-to-linecapacitance) is small, so that the delay in the transmittance of theelectric signal through the circuit (i.e., interconnect delay) is small,wherein the damascene process is commercially advantageous not only inthat a low resistance metal, such as copper or silver, can be used as amaterial for a circuit, but also in that this process is suitable forthe production of a multilayer circuit board.

Another object of the present invention is to provide a multilayercircuit board and a semiconductor device, each comprising theabove-mentioned excellent circuit structure.

The foregoing and other objects, features and advantages of the presentinvention will be apparent from the following detailed description andclaims taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional view of a structure comprising a substratehaving formed thereon a silicon oxide-organic polymer composite thinfilm, and a photoresist film formed on the surface of the composite thinfilm;

FIG. 2 is a cross-sectional view of a structure having a photoresistpattern corresponding to the desired pattern of a circuit, which isobtained by exposing and developing the photoresist film of thestructure of FIG. 1;

FIG. 3 is a cross-sectional view of a structure having a groove defininga pattern for a circuit, which is obtained by etching the siliconoxide-organic polymer composite thin film of the structure of FIG. 2 inaccordance with the above-mentioned photoresist pattern;

FIG. 4 is a cross-sectional view of a structure obtained by removing thephotoresist from the structure of FIG. 3;

FIG. 5 is a cross-sectional view of a structure obtained by forming ametal layer on the surface of the structure of FIG. 4;

FIG. 6 is a cross-sectional view of a circuit structure obtained byremoving the metal which is not present in the groove of the structureof FIG. 5, or obtained by forming a metal layer only in the groove ofthe structure of FIG. 4 by selective plating method; and

FIG. 7 is a cross-sectional view of a circuit structure having aninsulator layer comprising a porous silicon oxide thin film, which isobtained by removing the organic polymer from the silicon oxide-organicpolymer composite thin film of the structure of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided a method forproducing a circuit structure comprising:

(a) a substrate,

(b) an insulator layer formed on the substrate, the insulator layercomprising a porous silicon oxide thin film having a groove, the groovedefining a pattern for a circuit, and

(c) a circuit comprising a metal layer formed in the groove,

the method comprising the steps of:

(1) forming a preliminary insulator layer comprising a siliconoxide-organic polymer composite thin film formed on the substrate, thesilicon oxide-organic polymer composite thin film comprising a siliconoxide having an organic polymer dispersed therein,

(2) forming, in the preliminary insulator layer, a groove which definesa pattern for a circuit,

(3) forming, in the groove, a metal layer which functions as a circuit,and

(4) removing the organic polymer from the silicon oxide-organic polymercomposite thin film of the preliminary insulator layer to render thepreliminary insulator layer porous, thereby converting the preliminaryinsulator layer to an insulator layer comprising a porous silicon oxidethin film.

For easy understanding of the present invention, the essential featuresand various preferred embodiments of the present invention areenumerated below.

1. A method for producing a circuit structure comprising:

(a) a substrate,

(b) an insulator layer formed on the substrate, the insulator layercomprising a porous silicon oxide thin film having a groove, the groovedefining a pattern for a circuit, and

(c) a circuit comprising a metal layer formed in the groove,

the method comprising the steps of:

(1) forming a preliminary insulator layer comprising a siliconoxide-organic polymer composite thin film formed on the substrate, thesilicon oxide-organic polymer composite thin film comprising a siliconoxide having an organic polymer dispersed therein,

(2) forming, in the preliminary insulator layer, a groove which definesa pattern for a circuit,

(3) forming, in the groove, a metal layer which functions as a circuit,and

(4) removing the organic polymer from the silicon oxide-organic polymercomposite thin film of the preliminary insulator layer to render thepreliminary insulator layer porous, thereby converting the preliminaryinsulator layer to an insulator layer comprising a porous silicon oxidethin film.

2. The method according to item 1 above, wherein the removal of theorganic polymer from the preliminary insulator layer is conducted bylight irradiation-heat treatment.

3. The method according to item 1 or 2 above, wherein the organicpolymer in the silicon oxide-organic polymer composite thin filmcomprises at least one polymer selected from the group consisting of apolymer having a graft structure and a polymer having athree-dimensional network structure.

4. A circuit structure produced by the method of any one of items 1 to 3above.

5. A multilayer circuit board comprising a laminate of a plurality ofcircuit structures, wherein at least one circuit structure of thecircuit structures is the circuit structure of item 4 above.

6. A semiconductor device comprising the circuit structure of item 4above.

The present invention will now be described in detail.

The circuit structure obtained by the method of the present inventioncomprises: (a) a substrate, (b) an insulator layer formed on thesubstrate, which comprises a porous silicon oxide thin film having agroove defining a pattern for a circuit, and (c) a circuit comprising ametal layer formed in the groove.

In the method of the present invention for producing a circuitstructure, first, a preliminary insulator layer is formed on asubstrate. The preliminary insulator layer comprises a siliconoxide-organic polymer composite thin film comprising a silicon oxidehaving an organic polymer dispersed therein.

Examples of substrates include a substrate made of a semiconductor, suchas silicon or germanium, a substrate made of a compound semiconductor,such as gallium-arsenic or indium-antimony, and a prepreg for thesubstrate of a printed circuit board. With respect to the thickness ofthe substrate, there is no particular limitation. For example, in thecase of a semiconductor substrate, the thickness of the substrate ispreferably 0.1 mm or more, more preferably from 0.3 to 1 mm. When thethickness of the semiconductor substrate is not more than 0.1 mm, thereis a danger that the substrate is warped or distorted by the internalstress of the preliminary insulator layer or the insulator layer(comprising the porous silicon oxide thin film), which is formed on thesubstrate. If desired, prior to the formation of the preliminaryinsulator layer on the substrate, the substrate may be coated with afilm of a substance, such as silicon dioxide, a silicon nitride,titanium nitride, tungsten nitride or the like.

Further, with respect to the substrate used in the present invention,the substrate may have formed thereon a device or a circuit structure(each produced using a conductive substance, a semiconductor, aninsulating substance and the like by the method of the present inventionand/or the conventional method).

With respect to the above-mentioned silicon oxide-organic polymercomposite thin film which is formed on the substrate as a preliminaryinsulator layer, such a composite thin film is already known. Forexample, such a composite thin film can be produced by the methoddescribed in J. Macromol. Sci.-Chem., A28(9), pp.817-829 (1991).However, it is preferred to produce the silicon oxide-organic polymercomposite thin film by a method comprising:

(1) applying onto a substrate a liquid mixture of (i) a silicon compoundselected from the group consisting of an alkoxysilane, and a mixture ofan alkoxysilane and a hydrolysis product thereof, and (ii) an organicmaterial selected from the group consisting of an organic polymer havingno polymerizable functional group, an organic polymer having at leastone polymerizable functional group and a polymerizable organic monomer,wherein the liquid mixture may be a solution of the mixture of (i) and(ii) in a solvent described below, to thereby form a thin film of themixture of the silicon compound and the organic material on thesubstrate,

(2) subjecting the silicon compound in the thin film to hydrolysis anddehydration condensation under conditions wherein the gelation of thesilicon compound satisfactorily proceed, to thereby gelate the siliconcompound in the thin film,

wherein, when the organic polymer of the mixture used for forming thethin film contains an organic polymer having at least one polymerizablefunctional group, a polymerizable organic monomer or a mixture thereof,the organic material present in the thin film is subjected topolymerization reaction before, simultaneously with or after thehydrolysis and dehydration condensation of the silicon compound,

thereby forming on the substrate a silicon oxide-organic polymercomposite thin film in which the particles of the organic polymer aredispersed in the silicon oxide.

However, the method for forming the silicon oxide-organic polymer thinfilm is not limited to the method exemplified above.

Hereinbelow, the above-mentioned method is explained.

It is preferred that the silicon compound used in the present inventionis an alkoxysilane or a mixture of an alkoxysilane and a hydrolysisproduct thereof. When such a silicon compound is used, theabove-mentioned thin film of the mixture of the silicon compound and theorganic material can be converted into the preliminary insulator layerby treating the thin film of the mixture of the silicon compound and theorganic material with water or steam. Specifically, when the thin filmcontaining an alkoxysilane as the silicon compound is treated with wateror steam, the alkoxy group of the alkoxysilane is converted to ahydroxyl group, thereby causing the dehydration condensation reaction ofthe silane, so that the gelation of the silicon compound occurs tothereby forming a preliminary insulator layer comprising the siliconoxide-organic polymer composite thin film having a structure in whichthe organic polymer is dispersed in the silicon oxide. When the mixtureof the silicon compound and the organic material is a solution thereofin a solvent, it is preferred that the amount of the solvent is 0.05% byweight or more, based on the weight of the solution. The thickness ofthe silicon oxide-organic polymer composite thin film is preferably 0.1to 10 μm, more preferably 0.2 to 3 μm. When the thickness of the siliconoxide-organic polymer composite thin film is more than 10 μm,unfavorable cracking may occur.

It is preferred that the organic polymer used in the present inventionis an organic polymer having no polymerizable functional group, anorganic polymer having at least one polymerizable functional group, apolymerizable organic monomer or a mixture thereof.

With respect to the organic polymer having no polymerizable functionalgroup, there is no particular limitation. Preferred examples of organicpolymers having no polymerizable functional group include polyethers,such as polyethylene glycol, polypropylene glycol and polytetramethyleneglycol; amides, such as a polyacrylamide derivative, apolymethacrylamide derivative, poly(N-vinylpyrrolidone) andpoly(N-acylethyleneimine); poly(vinyl alcohol); poly(vinyl acetate);esters, such as a poly(acrylic acid) derivative (e.g., poly(methylacrylate) and poly(ethyl acrylate)), a poly(methacrylic acid) derivative(e.g., poly(methyl methacrylate) and poly(ethyl methacrylate)) andpolycaprolactone; polyanhydride, such as poly(malonyl oxide),poly(adipoyl oxide), poly(pimeloyl oxide), poly(suberoyl oxide),poly(azelaoyl oxide) and poly(sebacoyl oxide); polymides; polyurethanes;polyureas; and polycarbonates. Further, as the organic polymer having nopolymerizable functional group, various copolymer can be also used.Examples of copolymers include a copolymer of the monomers used forproducing the above-mentioned polymers, such as an ethyleneglycol/propylene glycol copolymer, an acrylamide/acrylic acid copolymeror a vinyl alcohol/vinyl acetate copolymer; a copolymer of a monomerused in the production of the above-mentioned polymer and anothermonomer, such as an ethylene/acrylic acid copolymer or a vinylchloride/vinyl acetate copolymer. With respect to the degree ofpolymerization of the organic polymer having no polymerizable functionalgroup, it is generally in the range of from 8 to 350,000. As the organicpolymer having no polymerizable functional group, it is preferred to usean organic polymer formed from an alipahtic compound, since, asdescribed below, a preliminary insulator layer containing such anorganic polymer can be easily converted into the insulator layercomprising a porous silicon oxide thin film. From this point of view,especially preferred are polyethers, such as polyethylene glycol,polypropylene glycol and polytetramethylene glycol.

Further, from the viewpoint of improving the mechanical strength of thesilicon oxide-organic polymer composite thin film and suppressing theshrinkage of the composite thin film during the conversion thereof intothe porous silicon oxide thin film, it is preferred that the organicpolymer has at least one polymerizable functional group. In this case,the organic polymer forms a graft structure and/or a three-dimensionalnetwork structure in the silicon oxideorganic polymer composite thinfilm. Examples of polymerizable functional groups of the organic polymerinclude a vinyl group, a vinylidene group, a vinylene group, a glycidylgroup, an allyl group, an acrylate group, a methacrylate group, anacrylamide group, a methacrylamide group, a carboxyl group, a hydroxylgroup, an isocyanate group, an amino group, an imino group and ahalogen. Each of these polymerizable functional groups may be present atthe main chain, terminal or side chain of the polymer. Further, thefunctional groups may be bonded directly to the polymer or bonded to thepolymer through a spacer, such as an alkylene group and an ether group.The organic polymer may have a single type of the functional group or atleast two different types of the functional groups. Among the functionalgroups mentioned above, preferred are a vinyl group, a vinylidene group,a vinylene group, a glycidyl group, an allyl group, an acrylate group, amethacrylate group, an acrylamide group and a methacrylamide group.

Specific examples of preferred organic polymers having a polymerizablefunctional group include: an aliphatic polyether having at terminal(s)thereof a polymerizable functional group, such as an acrylate group, amethacrylate group, a vinyl group or a glycidyl group, which aliphaticpolyether is represented by polyethylene glycol acrylate, polyethyleneglycol diacrylate, polyethylene glycol methacrylate, polyethylene glycoldimethacrylate, polyethylene glycol alkyletheracrylate, polyethyleneglycol alkylethermethacrylate, polyethylene glycol vinyl ether,polyethylene glycol divinyl ether, polyethylene glycol glycidyl ether,polyethylene glycol diglycidyl ether, polypropylene glycol acrylate,polypropylene glycol diacrylate, polypropylene glycol methacrylate,polypropylene glycol dimethacylate, polypropylene glycolalkyletheracrylate, polypropylene glycol alkylethermethacrylate,polypropylene glycol vinyl ether, polypropylene glycol divinyl ether,polypropylene glycol glycidyl ether, polypropylene glycol diglycidylether and the like; a poly(meth)acrylate having at the side chainthereof a polymerizable functional group, such as a vinyl group, aglycidyl group and a allyl group, which poly(meth)acrylate isrepresented by poly(glycidyl acrylate), poly(glycidyl methacrylate),poly(allyl acrylate), poly(allyl methacrylate), poly(vinyl acrylate),poly(vinyl methacrylate) and the like; poly(vinyl cinnamate); and anepoxy resin. Among these, from the viewpoint of easiness in conversionof the silicon oxide-organic polymer composite thin film into the poroussilicon oxide thin film by the heat treatment described below,especially preferred are: polyethylene glycol acrylate, polyethyleneglycol diacrylate, polyethylene glycol methacrylate, polyethylene glycoldimethacrylate, polyethylene glycol alkyletheracrylate, polyethyleneglycol alkylethermethacrylate, polyethylene glycol vinyl ether,polyethylene glycol divinyl ether, polyethylene glycol glycidyl ether,polyethylene glycol diglycidyl ether, polypropylene glycol acrylate,polypropylene glycol diacrylate, polypropylene glycol methacrylate,polypropylene glycol dimethacrylate, polypropylene glycolalkyletheracrylate, polypropylene glycol alkylethermethacrylate,polypropylene glycol vinyl ether, polypropylene glycol divinyl ether,polypropylene glycol glycidyl ether and polypropylene glycol diglycidylether.

In the method of the present invention, when the organic polymer havinga polymerizable functional group and/or the organic polymer having nopolymerizable functional group is used as an organic material, theamount of the organic polymer used is generally 10⁻² to 100 parts byweight, preferably 10⁻¹ to 10 parts by weight, more preferably 10⁻¹ to 5parts by weight, per part by weight of the silicon compound. When theamount of the organic polymer used is less than 10⁻² part by weight, itbecomes likely that the porosity of the porous silicon oxide thin filmbecomes too small and, hence, a desired dielectric constant cannot beobtained. On the other hand, when the amount of the organic polymer usedis more than 100 parts by weight, it becomes likely that the strength ofthe porous silicon oxide thin film becomes low and, hence, the thin filmcannot be put into practical use.

In the method of the present invention, with respect to thepolymerizable organic monomer used as an organic material, there is noparticular limitation. When a bifunctional monomer is used, theresultant organic polymer forms a graft structure and/or athree-dimensional network structure in the silicon oxideorganic polymercomposite thin film.

Preferred examples of polymerizable organic monomers include acrylicacid, methacrylic acid and derivatives thereof, such as an acrylate, amethacrylate, ethylene bisacrylate, ethylene bismethacrylate,acyanoacrylic acid and a-cyanoacrylate; acid vinyl esters, such as vinylacetate, vinyl propionate, vinyl crotonate, vinyl benzoate and vinylchloroformate; amides, such as acrylamide, methacrylamide,N,N′-dialkylacrylamide, N,N′-dialkylmethacrylamide, N-alkylacrylamide,N-alkylmethacrylamide, N,N′-methylenebisacrylamide, N-vinylpyrrolidone,N-vinylformamide and N-vinylacetamide; vinyl group-containinghydrocarbons, such as styrene, α-methylstyrene, p-methoxystyrene,diphenylethylene, vinylnaphthalene, vinylanthracene, vinylcyclopentane,vinylcyclohexane and 5-vinyl-2-norbornene; acrylonitrile,methacrylonitrile and derivatives thereof; vinyl amines, such asN-vinylpyridine, N-vinylcarbazole and N-vinylimidazole;vinylalkylethers; vinylalkylketones; glycidyl acrylate; glycidylmethacrylate; and an epoxy resin.

With respect to the above-mentioned organic materials (i.e., the organicpolymers having no polymerizable functional group, the organic polymershaving a polymerizable functional group or the polymerizable organicmonomers), they can be used individually or in combination. Further, theorganic polymer and the organic monomer can be used in combination.

When a polymerizable organic monomer is used, the amount of thepolymerizable organic monomer is generally 10⁻² to 100 parts by weight,preferably 10⁻¹ to 10 parts by weight, more preferably 10⁻¹ to 5 partsby weight, per part by weight of the silicon compound. When theabove-mentioned organic polymer and the above-mentioned polymerizablemonomer are used in combination, the total amount of the organic polymerand the polymerizable monomer is generally in the same range asmentioned above.

As mentioned above, when the thin film of the mixture of the siliconcompound and the organic material contains the organic polymer having atleast one polymerizable functional group, the polymerizable organicmonomer or the mixture thereof, the organic material present in the thinfilm is subjected to polymerization reaction before, simultaneously withor after the hydrolysis and dehydration condensation of the siliconcompound.

In the method of the present invention, when the organic polymer havinga polymerizable functional group, the polymerizable organic monomer orthe mixture thereof is used as the organic material, a polymerizationinitiator can be used for accelerating the polymerization reaction. Asthe polymerization initiator, a known polymerization initiator can beused. Examples of known polymerization initiators include thermalinitiators, such as an azo compound and an organic peroxide;photoinitiators, such as a diazo compound, an azide and a derivative ofacetophenone; a photo acid generator; and a photo alkali generator.These initiators can be used individually or in combination. The thermalpolymerization and photopolymerization using the initiator can beperformed by a known method. The amount of the initiator used isgenerally 10⁻³ part by weight, preferably 10⁻² to 10⁻¹ part by weight,per part by weight of the organic polymer having a polymerizablefunctional group and/or the polymerizable organic monomer.

Preferred examples of alkoxysilanes usable as the silicon compound inthe method of the present invention include tetraalkoxysilanes, such astetramethyoxysilane, tetraethoxysilane, tetra(n-propoxy)silane,tetra(i-propoxy)silane, tetra(n-butoxy)silane and tetra(t-butoxy)silane.Further examples of alkoxysilanes usable in the present inventioninclude an oligomer of an alkoxysilane, such as the so-called “ethylsilicate” or “methyl silicate”; an alkoxysilane having bonded to thesilicon atom thereof a hydrogen atom, an alkyl group or an aryl group,such as trimethoxysilane, triethoxysilane, methyltrimethoxysilane,methyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane,bis(trimethoxysilyl)methane, bis(triethoxysilyl)methane,1,2-bis(trimethoxysilyl)ethane, 1,2-bis(triethoxysilyl)ethane,1,4-bis(trimethoxysilyl)benzene or 1,4-bis(triethoxysilyl)benzene; analkoxysilane having the alkoxy group thereof replaced with a hydroxylgroup; and a hydrolysis product of an alkoxysilane, in which thealkoxysilane is oligomerized. These alkoxysilanes can be usedindividually or in combination.

Further, from the viewpoint of controlling the properties of each of thepreliminary insulator layer (comprising a silicon oxide-organic polymercomposite thin film) and the porous silicon oxide thin film, amonoalkoxysilane or a dialkoxysilane, which has bonded to the siliconatom thereof 2 or 3 non-alkoxy substituents selected from the groupconsisting of a hydrogen atom, an alkyl group and an aryl group, can beused in combination with the alkoxysilanes mentioned above. The amountof the monoalkoxysilane and/or the dialkoxysilane is generally 80 mol %or less, based on the total molar amount of the silicon compounds. Whenthe amount of the monoalkoxysilane and/or the dialkoxysilane exceeds 80mol %, there is a danger that the gelation of the silicon compound doesnot occur.

In the method of the present invention, it is not necessary to dissolvethe silicon compound/organic material mixture (used for forming thesilicon oxideorganic polymer composite thin film) in a solvent. However,the compatibility between an alkoxysilane and an organic polymer isgenerally poor. Therefore, when the compatibility between the siliconcompound used and the organic material used is poor, it is preferred touse a solvent capable of dissolving therein both of them. On the otherhand, as specific examples of combinations of the silicon compound andthe organic material which do not require the use of a solvent, therecan be mentioned a combination of a polyethylene glycol (number averagemolecular weight: 400 to 1,000), which is in the liquid state, andtetraethoxysilane, and a combination of N,N-dimethylacrylamide(monomer), which is in the liquid state, and tetraethoxysilane. In eachof the above-mentioned combinations, the uniform liquid mixture of thesilicon compound and the organic material can be obtained without theuse of a solvent.

With respect to the above-mentioned solvent, there is no particularlimitation, as long as it is possible to obtain a solution havingdissolved therein both the silicon compound and the organic material.Specifically, for example, when an alkoxysilane is used as the siliconcompound, it is possible to use a solvent which is not capable ofdissolving therein the alkoxysilane as such, but is capable ofdissolving therein a mixture of the alkoxysilane and a hydrolysisproduct thereof, which is obtained by subjecting the alkoxysilane topartial hydrolysis. For example, when tetraethoxysilane is added to amixed solvent of DMF (N,N-dimethylformamide) and ethanol, thetetraethoxysilane is not dissolved in the mixed solvent, and theresultant mixture separates into two layers. However, when a smallamount of diluted hydrochloric acid is added to the obtained mixture andthen, the resultant mixture is vigorously stirred to thereby subjectingthe tetraethoxysilane to partial hydrolysis, a homogeneous solution isobtained in 2 to 3 minutes after the start of the partial hydrolysis.Therefore, in the present invention, it is possible to use a combinationof the alkoxysilane, the organic polymer and the solvent, which can beused for obtaining the solution in the above-mentioned manner.

Preferred examples of solvents include alcohols, such as a C₁-C₄monohydric alcohol, a C₁-C₄ dihydric alcohol and glycerol; amides, suchas formamide, N-methylformamide, N-ethylformamide,N,N-dimethylformamide, N,N-diethylformamide, N-methylacetamide,N-ethylacetamide, N,N-dimethylacetamide, N,N-diethylacetamide,N-methylpyrrolidone, N-formylmorpholine, N-acetylmorpholine,N-formylpiperidine, N-acetylpiperidine, N-formylpyrrolidine,N-acetylpyrrolidine, N,N′-diformylpiperazine andN,N′-diacetylpiperazine; urea and derivatives thereof, such astetramethylurea and N,N′-dimethylimidazolidinone; ethers, such astetrahydrofuran, diethyl ether, di(npropyl) ether, diisopropyl ether,diglyme, 1,4-dioxane, ethylene glycol monomethyl ether, ethylene glycoldimethyl ether, ethylene glycol diethyl ether, propylene glycolmonomethyl ether and propylene glycol dimethyl ether; esters, such asethyl formate, methyl acetate, ethyl acetate, ethyl lactate, ethyleneglycol monomethyl ether acetate, ethylene glycol diacetate, propyleneglycol monomethyl ether acetate, diethyl carbonate, ethylene carbonateand propylene carbonate; ketones, such as acetone, methyl ethyl ketone,methyl propyl ketone, methyl n-butyl ketone, methyl isobutyl ketone,methyl amyl ketone, cyclopentanone and cyclohexanone; nitriles, such asacetonitrile, propiononitrile, n-butyronitrile and isobutyronitrile; anddimethyl sulfoxide, dimethyl sulfone, and sulfolane. These solvents canbe used individually or in combination. Further, each of the abovesolvents can be mixed with a solvent other than mentioned above, or anadditive, such as a leveling agent (which is used for improving theuniformity in the thickness of a coating), an adhesion promoter, apolymerization initiator, an acid generator, an alkali generator or anoxidizing agent.

Among the solvents mentioned above, especially preferred are amides,such as formamide, N-methylformamide, N-ethylformamide,N,N-dimethylformamide, N,N-diethylformamide, N-methylacetamide,N-ethylacetamide, N,N-dimethylacetamide, N,N-diethylacetamide,N-methylpyrrolidone, N-formylmorpholine, N-acetylmorpholine,N-formylpiperidine, N-acetylpiperidine, N-formylpyrrolidine,N-acetylpyrrolidine, N,N′-diformylpiperazine andN,N′-diacetylpiperazine; and urea and derivatives thereof, such astetramethylurea and N,N′-dimethylimidazolidinone. These solvents can beadvantageously used for obtaining a transparent and uniform siliconoxide-organic polymer composite thin film and a porous silicon oxidethin film having small-size pores.

In the method of the present invention, the hydrolysis and dehydrationcondensation of the silicon compound for forming the siliconoxide-organic polymer composite thin film need not be conducted in thepresence of a catalyst. However, a catalyst may be used for promptingthe hydrolysis and dehydration condensation of the silicon compound.Specific examples of catalysts for the hydrolysis and dehydrationcondensation of the silicon compound include acids, such as hydrochloricacid, nitric acid, sulfuric acid, formic acid, acetic acid, oxalic acidand maleic acid; alkali compounds, such as aqueous ammonia, potassiumhydroxide, sodium hydroxide, triethylamine, triethanolamine, pyridine,piperidine and choline. These compounds can be used individually or incombination. Further, two or more of these compounds can be used in astepwise manner. The term “stepwise manner” herein is intended to meanthat, for example, the silicon compound/organic material mixture coatedon the substrate is treated with an acid catalyst, followed by theaddition of an alkali catalyst, and vice versa.

The amount of the catalyst for the hydrolysis and dehydrationcondensation of the silicon compound is generally 1 mol or less,preferably 10⁻¹ mol or less, per mol of the silicon compound. When theamount of the catalyst exceeds 1 mol, a disadvantage is likely to becaused that the precipitation or the like occurs and hence, a uniformporous silicon oxide thin film cannot be obtained.

The hydrolysis of the alkoxysilane in the thin film of the siliconcompound/organic material mixture can be effected as follows. When theabove-mentioned catalyst is used in the form of an aqueous solutionthereof, the hydrolysis can be effected by utilizing the water used as asolvent for the catalyst. Further, even if water is not added to thesilicon compound/organic material mixture, the moisture present in theair can be utilized for effecting the hydrolysis if the amount of themoisture present in the air is sufficient for effecting the hydrolysis.In such a case, if desired, water may be further added to the siliconcompound/organic material mixture. The amount of water to be added ispreferably 0.3 to 10⁴ mol, more preferably 1 to 10 mol, per mol of thesilicon atom contained in the alkoxysilane. When the amount is more than10⁴ mol, the uniformity of the silicon oxide-organic polymer compositethin film is likely to be lowered.

With respect to the method for forming the thin film of the siliconcompound/organic material mixture by applying onto a substrate thesilicon compound/organic material mixture which is in the form of aliquid or a solution, a conventional method, such as a flow castingmethod, a revolution method and an immersion method, can be employed.The surface of the substrate may be preliminarily treated with anadhesion promoter. As the adhesion promoter, the so-calledsilane-coupling agent, an aluminum chelate compound and the like can beused. Examples of especially preferred adhesion promoters include3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane,N-2-(aminoethyl)-3-aminopropyltrimethoxysilane,N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane,vinyltrichlorosilane, vinyltriethoxysilane,3-chloropropyltrimethoxysilane, 3-chloropropylmethyldichlorosilane,3-chloropropylmethyldimethoxysilane, 3-chloropropyldiethoxysilane,3-mercaptopropyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane,3-glycidoxypropylmethyldimethoxysilane,3-methcryloxypropyltrimethoxysilane,3-methacryloxypropylmethyldimethoxysilane, hexamethyldisilazane,ethylacetoacetatealuminumdiisopropyrate,aluminum-tris(ethylacetoacetate),aluminum-bis(ethylacetoacetate)monoacetylacetonate andaluminum-tris(acetylacetonate). If desired, these adhesion promoters maybe used in combination with other additives, such as water, an acid andan alkali, or may be diluted with a solvent. The treatment with theadhesion promoter can be effected by a known method, for example, by amethod described in a catalogue of a commercially availablesilane-coupling agent and the like.

With respect to the temperature for the gelation of the siliconcompound, there is no particular limitation. The temperature isgenerally within the range of from 0 to 18° C., preferably from 30 to150° C. When the temperature is too low, the rate of gelation is likelyto become too low, so that it takes a long time to crosslink the siliconcompound to a satisfactory level. On the other hand, when thetemperature is too high, voids tend to be formed in the siliconoxideorganic polymer composite thin film, so that the uniformity of theobtained porous silicon oxide thin film is lowered.

The gelation time varies depending on the gelation temperature, theamount of a catalyst used for the gelation and the like. The gelationtime is generally within the range of from several minutes to severaldays.

When the organic polymer having a polymerizable functional group, thepolymerizable organic monomer or the mixture thereof is used, thepolymerization thereof can be promoted by heating. The temperature forheating can be selected within the range of from 20 to 200° C. dependingon the type of the polymerizable functional group contained in theorganic polymer and the organic monomer. When the polymerizablefunctional group contained in the organic polymer used or thepolymerizable organic monomer used is photopolymerizable, thepolymerization reaction can be effected by light irradiation. When apolymerization initiator is used, the polymerization can be promoted bya known method, such as heating and light irradiation, depending on thetype of the initiator. When the organic polymer having aphotopolymerizable functional group and/or the photopolymerizableorganic monomer, and the photopolymerization initiator are used, it ispossible to effect the polymerization at only a desired portion of thethin film of the silicon compound/organic material mixture by lightirradiation through a mask having a desired shape.

In the method of the present invention, when the organic polymer havinga polymerizable functional group and/or the polymerizable monomer isused as the organic material, the hydrolysis and dehydrationcondensation of the silicon compound can be performed before,simultaneously with, or after the polymerization reaction of the organicpolymer and/or the organic monomer, depending on the type and amount ofthe polymerization catalyst used and the polymerization initiator used,and the reaction conditions.

In the method of the present invention, when the silicon oxide-organicpolymer composite thin film is formed by effecting the gelation of thesilicon compound and the polymerization reaction of the organic polymerhaving a polymerizable functional group, the polymerizable organicmonomer or the mixture thereof using a solvent in a closed system, theobtained silicon oxide-organic polymer composite thin film contains thesolvent used. Therefore, the solvent is subsequently removed by drying.The drying temperature varies depending on the type of the solvent.However, the drying temperature is generally in the range of from 30 to250° C. When the evaporation of a solvent is slow and the remainingsolvent may cause a problem, such as lowering of the strength of thecomposite thin film, and contamination of the apparatus used in thesubsequent step, it is also preferred to effect the drying under reducedpressure for completely removing the solvent by volatilization. Further,from the view-point of obtaining a uniform composite thin film bysuppressing the occurrence of the formation of voids in the compositethin film, it is also preferred to gradually elevate the dryingtemperature.

On the other hand, when the formation of the silicon oxide-organicpolymer composite thin film is conducted using a solvent in an opensystem, the evaporation of the solvent occurs simultaneously with thegelation of the silicon compound and the polymerization reaction of theorganic polymer having a polymerizable functional group, thepolymerizable organic monomer or the mixture thereof. The timing of theevaporation of the solvent can be controlled by selecting the types ofthe silicon compound, the polymerizable functional group contained inthe organic polymer, the amount of the solvent, the vapor pressure ofthe solvent, the atmosphere and the like. However, under conditionsgenerally employed, the solvent is substantially completely removed(evaporated) at the completion of the gelation of the silicon compoundand the polymerization reaction of the organic polymer and/or theorganic monomer.

The thus obtained preliminary insulator layer comprising a siliconoxide-organic polymer composite thin film comprises a silicon oxidehaving dispersed therein an organic polymer.

In the obtained preliminary insulator layer, as described below indetail, a groove which defines a desired pattern for a circuit is formedand then, a metal layer which functions as a circuit is formed in thegroove. Then, the organic polymer is removed from the siliconoxide-organic polymer composite thin film of the preliminary insulatorlayer by the below-described heat treatment and the like to render thepreliminary insulator layer porous, thereby converting the preliminaryinsulator layer to an insulator layer comprising a porous silicon oxidethin film. Thus, the circuit structure comprising a substrate havingformed thereon an insulator layer is obtained.

With respect to the method for forming a groove (which defines a desiredpattern for a circuit) in the preliminary insulator layer, it isespecially preferred to employ a microprocessing method using aconventional lithography. Hereinbelow, an explanation is made withrespect to the method of the present invention using a conventionallithography, referring to FIGS. 1 to 7.

First, a photoresist is applied onto preliminary insulator layer 2comprising a silicon oxide-organic polymer composite thin film which isformed on substrate 1, thereby obtaining a structure as shown in FIG. 1,which has photoresist layer 3 formed on the preliminary insulator layer.The obtained photoresist layer 3 is exposed to light through a maskhaving a desired pattern to thereby form a latent image on thephotoresist layer, followed by development, thereby forming onpreliminary insulator layer 2 the photoresist image of the desiredpattern (see FIG. 2). With respect to the type of the photoresist, themethod for the light exposure and the method for the development, thereis no particular limitation, and the conventional photoresist and theconventional method can be employed. As an example of a commerciallyavailable photoresist which can be used in the present invention, therecan be mentioned photoresists manufactured and sold by Tokyo Ohka KogyoCo., Ltd., Japan under the trade name “TDUR”. Examples of lights usedfor the light exposure include KrF excimer laser, g-line and i-line of amercury lamp, ArF excimer laser and the like. As an example of adeveloper used in the development, there can be mentioned an aqueoussolution of tetramethylammonium hydroxide. Subsequently, as shown inFIG. 3, a portion of preliminary insulator layer 2 (comprising a siliconoxide-organic polymer composite thin film) which is not protected by thephotoresist layer is removed by etching, thereby forming a groovedefining a pattern for a circuit on preliminary insulator layer 2.Examples of methods for etching include a plasma etching, a reactive ionetching, a down flow etching and a sputter etching. When the developerused for the development conducted after the light exposure for thepattern formation can dissolve preliminary insulator layer 2, theetching mentioned above can be effected by washing with the developer.Then, the photoresist is removed by washing with a solvent (aphotoresist remover), plasma irradiation and the like to obtain astructure as shown in FIG. 4.

Then, as shown in FIG. 5, metal 4 functioning as a circuit is depositedin the groove and on the surface of preliminary insulator layer 2.Preferred examples of metals include aluminum, copper, silver, tungstenand titanium. These metals can be used individually or in combination.Prior to the deposition of the metal, a barrier layer may be formed onthe surface of preliminary insulator layer 2 and the side walls andbottom wall of the groove, so that metal 4 is deposited in the grooveand on the surface of the preliminary insulator layer through thebarrier layer. Examples of materials for forming the barrier layerinclude an electrically conductive material, such as titanium nitride,and an electrically insulating material, such as a silicon nitride or asilicon oxide. Each of the deposition of metal 4 and the formation ofthe barrier layer can be conducted by a conventional method, such as aphysical deposition method (e.g., the sputtering) or an electricaldeposition method (e.g., the CVD (chemical vapor deposition), theelectroplating or the electroless plating). As another example ofmethods for the deposition of metal 4, there can be mentioned a methodin which a solution of a metal-organic material composite(metallo-organics) (in which a metal and an organic material arechemically bonded to each other), followed by calcination. As an exampleof such a solution of the metal-organic material composite, there can bementioned “Metallo-organics” manufactured and sold by Tanaka KikinzokuKogyo K.K., Japan.

Then, as shown in FIG. 6, the excess of the deposited metal 4 (i.e., themetal deposited on the portion other than the groove) is removed. As apreferred method for removing the excess of the deposited metal 4, therecan be mentioned an etch back method using plasma and the like, and achemical mechanical polishing (CMP) method. The CMP method is especiallypreferred.

Further, when the selective plating method is used for depositing metal4 in the groove of preliminary insulator layer 2, it is possible todeposit metal 4 selectively in the groove of preliminary insulator layer2. Therefore, in this case, the structure as shown in FIG. 6 can beprepared directly from the structure as shown in FIG. 4, i.e., it is notnecessary form a structure as shown in FIG. 5 before the formation ofthe structure as shown in FIG. 6. With respect to the selective platingmethod, reference can be made to, for example, WO98/40910.

Then, from the silicon oxide-organic polymer composite posite thin filmof preliminary insulator layer 2, the organic polymer is removed tothereby obtain the circuit structure of the present invention having astructure as shown in FIG. 7, which comprises a substrate 1, insulatorlayer 5 (formed on substrate 1) comprising a porous silicon oxide thinfilm having a groove, and a circuit comprising a metal layer formed inthe groove. Examples of methods for removing the organic polymer includea solvent extraction and a plasma treatment. Further, as the simplestmethod, there can be mentioned a method in which the siliconoxide-organic polymer composite thin film is heated at a temperatureequal to or higher than the decomposition temperature of the organicpolymer contained in the composite thin film for 1 minute to severaldays, thereby decomposing the organic polymer. Needless to say, thetemperature for decomposing the organic polymer should be selected,taking into consideration the types of the metal used and the organicmaterial used. Generally, the temperature for decomposing the organicpolymer is equal to or higher than the decomposition temperature of theorganic polymer used, and is lower than the temperature at which themetal layer (functioning as a circuit) is melted. From the viewpoint ofpreventing the damage to an element or a circuit, which has been alreadyformed on the substrate, the temperature for decomposing the organicpolymer is preferably within the range of from 100 to 450° C., morepreferably from 150 to 400° C. From this viewpoint, it is preferred thatthe heat treatment is conducted by light irradiation. In this case, itis especially preferred to use a light having such wavelengths that thepreliminary insulator layer (comprising a silicon oxide-organic polymercomposite thin film) can absorb the light.

When the polymer having a polymerizable functional group is used as theorganic material and a photopolymerization initiator is used forpromoting the polymerization of the polymer, the method of the presentinvention can be carried out without using a photoresist. Specifically,in this case, a structure as shown in FIG. 4, which has a preliminaryinsulator layer having a groove defining a desired pattern, can beobtained by a method comprising applying onto a substrate a raw materialmixture containing a silicon compound, an organic polymer having apolymerizable functional group and a photopolymerization initiator,thereby forming a preliminary insulator layer on the substrate; exposingthe preliminary insulator layer to light through a mask having a desiredpattern, thereby polymerizing the polymer at the exposed portions of thepreliminary insulator layer; and subjecting the resultant preliminaryinsulator layer to development (i.e., removal of the polymer at theunexposed portions of the insulator layer). The subsequent steps can beconducted in substantially the same manner as mentioned above.

In the present invention, it is preferred to treat the obtained poroussilicon oxide thin film with a silylation agent for suppressing thewater absorption of the thin film and improving the adhesion of the thinfilm to other substances. Examples of silylation agents usable in thepresent invention include alkoxysilanes, such as trimethylmethoxysilane,trimethylethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane,methyltrimethoxysilane, methyltriethoxysilane, dimethylethoxysilane,methyldiethoxysilane, dimethylvinylmethoxysilane,dimethylvinylethoxysilane, diphenyldimethoxysilane,diphenyldiethoxysilane, phenyltrimethoxysilane andphenyltriethoxysilane; chlorosilanes, such as trimethylchlorosilane,dimethyldichlorosilane, methyltrichlorosilane, methyldichlorosilane,dimethylchlorosilane, dimethylvinylchlorosilane,methylvinyldichlorosilane, methylchlorodisilane, triphenylchlorosilane,methyldiphenylchlorosilane and diphenyldichlorosilane; and silazanes,such as hexamethyldisilazane, N,N′-bis(trimethylsilyl)urea,N-trimethylsilylacetamide, dimethyltrimethylsilylamine,diethyltriethylsilylamine and trimethylsilylimidazole. The treatmentwith the silylation agent can be conducted by a known method, such ascoating, immersion and vapor exposure.

According to the method of the present invention, a circuit structurehaving an insulator layer comprising a porous silicon oxide having a lowdielectric constant can be very easily produced by the damascene processwhich is advantageous not only in that a low resistance metal, such ascopper or silver, can be used as a material for a circuit, but also inthat this process is suitable for producing a multi-layer circuitstructure. Furthermore, the method of the present invention is free froma problem, such as the damage to the insulator layer (comprising aporous silicon oxide thin film) and the entrapment of an etching gas,metal particles and the like in the pores of the insulator layer. By themethod of the present invention, not only can the line-to-linecapacitance in the circuit structure be lowered, but also a lowresistance metal, such as copper or silver, can be used as a materialfor a circuit, so that it has become possible to produce an excellentcircuit structure in which the interconnect delay is greatly suppressed,as compared to the case of the conventional circuit structures.

The circuit structure obtained by the method of the present inventioncan be very advantageously used for producing a multi-layer circuitboard and a semi-conductor device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be described in more detail withreference to the following Examples and Comparative Examples, but theyshould not be construed as limiting the scope of the present invention.

EXAMPLE 1

0.74 g of methyltriethoxysilane, 2.4 g of tetraethoxysilane, 0.68 g ofpolyethylene glycol monomethacrylate having a number average molecularweight of 360 and 0.34 g of polyethylene glycol dimethacrylate having anumber average molecular weight of 540 were dissolved in a mixed solventof 2.0 g of N-methylpyrrolidone and 1.0 g of propylene glycol methylether acetate. To the resultant solution were added 0.75 g of water and0.15 g of 0.1 N nitric acid, followed by stirring at room temperaturefor 2 hours. Subsequently, 0.05 g of dicumyl peroxide was added to thesolution. The resultant solution was coated onto a silicon wafer havingon its surface a thermal oxidation product film (an SiO₂ film) having athickness of 1.2 μm using a spin coater (trade name: 1H-360S SpinCoater, manufactured and sold by Mikasa Co., Ltd., Japan) underconditions wherein the revolution rate of the silicon wafer was 1,500rpm and the coating time was 10 seconds. The resultant coating washeated in the air at 120° C. for one hour and, then, at 180° C. for onehour, thereby obtaining a preliminary insulator layer comprising asilicon oxide-organic polymer composite thin film.

Then, a photoresist (trade name: THMR-iP3650, manufactured and sold byTokyo Ohka Kogyo Co., Ltd., Japan) was applied onto the obtainedpreliminary insulator layer to obtain a laminate structure as shown inFIG. 1 which has a photoresist layer having a thickness of 1.05 μm. Atest pattern image was transferred to the photoresist layer by means ofa light exposure apparatus (trade name: i-line stepper FPA300i4,manufactured and sold by CANON INC., Japan). The resultant photoresistlayer was developed using a 2.38% aqueous solution oftetramethylammonium hydroxide to remove portions of the photoresistlayer which had been exposed, thereby obtaining a structure as shown inFIG. 2 which has, on the surface of the preliminary insulator layer, aphotoresist image corresponding to the test pattern image. Theconfiguration of the test pattern will be explained below.

The preliminary insulator layer (comprising the silicon oxide-organicpolymer composite thin film) having the obtained photoresist image wassubjected to etching through the photoresist image as a mask (i.e., aprotective layer), using a reactive ion etcher (trade name: DEA506,manufactured and sold by Anelva Corporation, Japan), thereby removingportions of the silicon oxide-organic polymer composite thin film whichwere not protected by the mask to obtain a structure as shown in FIG. 3.The obtained structure had a positive pattern inversely corresponding tothe test pattern, which is defined by grooves formed in the siliconoxide-organic polymer composite thin film. The etching was conductedunder conditions wherein the total pressure in the etcher was adjustedto 30 Pa by using, as an etching gas, a gaseous mixture of 100 SCCM(“SCCM” is an abbreviation for “standard cubic centimeter per minute”which means a flow rate of a gas, as measured in terms of the volume ofthe gas flowed per minute under standard conditions (0° C., 1 atm)) ofcarbon tetrafluoride and 10 SCCM of oxygen, the electrical power was 300W, and the etching time was 20 minutes.

Then, the photoresist was completely removed by oxidation using an asher(trade name MPC600, manufactured and sold by Mori Engineering Co., Ltd.,Japan) under conditions wherein the oxygen pressure was 50 Pa, thetemperature was 50° C., the electrical power was 150 W, and theoxidation time was 15 minutes, thereby obtaining a structure as shown inFIG. 4.

On the surface of the obtained structure having a positive patterninversely corresponding to the test pattern, a titanium nitride filmhaving a thickness of 30 nm was formed using a sputtering apparatus(trade name: SPF313H, manufactured and sold by Anelva Corporation,Japan), and then, a copper film having a thickness of 50 nm was formedon the titanium nitride film using the above-mentioned sputteringapparatus. Specifically, the above-mentioned titanium nitride film wasformed by reactive sputtering using a titanium target and a gaseousmixture of argon and nitrogen (total pressure=0.27 Pa, partial pressureratio=50:50), and the above-mentioned copper film was formed using acopper target and argon (total pressure=0.27 Pa). In the formation ofboth films, the electrical power used was 400 W. The titanium target andthe copper target comprise copper plates respectively having solderedthereon a rolled titanium sheet and a rolled copper sheet, each having adesired shape. With respect to the sputtering, it is generally knownthat, during the sputtering, the ionized gas (such as argon gas) isforced to collide with the target to thereby expel the metal atoms orclusters thereof (i.e., titanium or copper atoms, or clusters thereof)from the surface of the target, and the expelled metal atoms or clustersthereof are deposited onto a substrate (such as a silicon wafer).

Then, the surface of the thus formed copper film was electroplated withcopper using copper sulfate and an aqueous solution of sulfuric acid, tothereby obtain a structure as shown in FIG. 5 which has a copper layerhaving a thickness of 1.2 μm. With respect to the obtained structure,the grooves defining the positive pattern inversely corresponding to thetest pattern (which were formed in the silicon oxide-organic polymercomposite thin film) were completely filled with copper, and portionsother than the grooves were also covered with a copper layer having athickness of about 1.2 μm. The surface of the structure having thecopper layer was polished with a slurry comprising aluminum oxide andhydrogen peroxide by means of a chemical mechanical polishing apparatus(trade name: 500STZ-6, manufactured and sold by NANOTECH MACHINES CO.,LTD., Japan) to thereby completely remove the extra copper deposited onthe portions other than the grooves. The resultant had a structure asshown in FIG. 6 which has copper layers in such a form as embedded inthe silicon oxide-organic polymer composite thin film. The polishing wasconducted under conditions wherein the polishing pressure was 4 Psi, therevolution rate of the wafer was 30 rpm, and the polishing time was 60seconds.

Finally, the thus obtained structure was subjected to heat treatment at400° C. in a nitrogen atmosphere for one hour to thereby burn off onlythe organic polymer in the preliminary insulator layer made of a siliconoxide-organic polymer composite thin film, thereby obtaining a circuitstructure as shown in FIG. 7 which has an insulator layer comprising aporous silicon oxide thin film.

The positive pattern inversely corresponding to the above-mentioned testpattern comprises two parallel grooves each having a width of 0.5 μm,wherein the distance between the two grooves is 0.3 μm. In the obtainedcircuit structure, the metal layers formed in grooves formed a metalliccircuit, and the insulator layer was present between the metal layers.Each of the metal layers (i.e., circuit lines) had an electrode padportion at a terminal thereof. To the obtained circuit structure wasapplied an alternating voltage through the electrode pad portions of themetal layers, and the capacitance of the metal layers and the insulatorlayer positioned therebetween (which can be regarded as the capacitanceof a parallel plate capacitor composed of the metals layers and theinsulator layer positioned therebetween) was measured as theline-to-line capacitance.

Specifically, with respect to the obtained circuit structure, theline-to-line capacitance was measured by means of a RFimpedance/material analyzer (trade name: HP4291A, manufactured and soldby Hewlett Packard Company, U.S.A.). As a result, it was found that theline-to-line capacitance was 0.101 pF/mm.

Comparative Example 1

A circuit structure was produced in substantially the same manner as inExample 1, except that the heat treatment (at 400° C. in a nitrogenatmosphere for one hour) for removing the organic polymer from thepreliminary insulator layer was not carried out. In the obtained circuitstructure, a preliminary insulator layer (comprising a siliconoxide-organic polymer composite thin film) as such was used as aninsulator layer.

With respect to the obtained circuit structure, the line-to-linecapacitance was measured. As a result, it was found that theline-to-line capacitance was 0.118 pF/mm. From the comparison betweenthe results of Example 1 and the results of Comparative Example 1, it isapparent that, by the use of the porous silicon oxide thin film(obtained by removing the organic polymer from the silicon oxide-organicpolymer composite thin film) as an insulator layer of a circuitstructure, the line-to-line capacitance can be lowered, as compared tothat in the case where the silicon oxide-organic polymer composite thinfilm as such is used as an insulator layer of a circuit structure.

Comparative Example 2

A preliminary insulator layer comprising a silicon oxide-organic polymercomposite thin film was formed on a silicon wafer in substantially thesame manner as in Example 1, except that the conditions for coatingusing a spin coater were changed, so that the thickness of the compositethin film became 0.7 μm instead of 0.8 μm. Then, the formed preliminaryinsulator layer was subjected to heat treatment for removing the organicpolymer under the same conditions as in Example 1 (i.e., at 400° C. in anitrogen atmosphere for one hour) to thereby burn off only the organicpolymer in the silicon oxide-organic polymer composite thin film,thereby obtaining an insulator layer comprising a porous silicon oxidethin film. On the obtained insulator layer was formed a film of silicondioxide having a thickness of 0.1 μm by chemical vapor deposition. Then,the formation of a positive pattern inversely corresponding to the testpattern (which is defined by grooves formed in the insulator layer), theformation of a copper layer and the removal of an extra copper (i.e.,copper deposited on the portion other than the grooves) were conductedin substantially the same manner as in Example 1, thereby obtaining acircuit structure. During the production of the circuit structure, carewas taken so as to form a positive pattern inversely corresponding tothe test pattern, which has substantially the same configuration as thatof the positive pattern in Example 1.

With respect to the obtained circuit structure, the line-to-linecapacitance was measured. As a result, it was found that theline-to-line capacitance was 0.109 pF/mm. That is, despite that theporous silicon oxide thin film formed under substantially the sameconditions as in Example 1 was used in Comparative Example 2, theline-to-line capacitance of the circuit structure obtained inComparative Example 2 was disadvantageously high, as compared to that ofthe circuit structure obtained in Example 1. It is considered that sucha disadvantageously high line-to-line capacitance of the circuitstructure of Comparative Example 2 was caused due to the presence of thenon-porous silicon dioxide film, which has a high dielectric constant,wherein the silicon dioxide film was used for protecting the poroussilicon oxide thin film which was used as the insulator layer.

EXAMPLE 2

A circuit structure was produced in substantially the same manner as inExample 1, except that the removal of the organic polymer from thesilicon oxideorganic polymer composite thin film was conducted by lightirradiation at 380° C. in an argon atmosphere at 1 atm for 30 minutesusing an infrared lamp, instead of heat treatment at 400° C. in anitrogen atmosphere for one hour.

With respect to the obtained circuit structure, the line-to-linecapacitance was measured. As a result, it was found that theline-to-line capacitance was 0.102 pF/mm. This value is almost the sameas that (0.101 pF/mm) in Example 1. This shows that, even when theremoval of the organic polymer from the silicon oxide-organic polymercomposite thin film is conducted by light radiation, it is possible toachieve substantially the same effect as in the case where the removalof the organic polymer was conducted by the general heat treatment asconducted in Example 1 (i.e., heat treatment at 400° C. for one hour).

Reference Example 1

0.74 g of methyltriethoxysilane, 2.4 g of tetraethoxysilane, 0.68 g ofpolyethylene glycol monomethacrylate having a number average molecularweight of 360 and 0.34 g of polyethylene glycol dimethacrylate having anumber average molecular weight of 540 were dissolved in a mixed solventof 2.0 g of N-methylpyrrolidone and 1.0 g of propylene glycol methylether acetate. To the resultant solution were added 0.75 g of water and0.15 g of 0.1 N nitric acid, followed by stirring at room temperaturefor 2 hours. Subsequently, 0.05 g of dicumyl peroxide was added to thesolution. The resultant solution was coated onto a silicon wafer havingon its surface a titanium nitride thin film, using a spin coater (tradename: 1H-360S Spin Coater, manufactured and sold by Mikasa Co., Ltd.,Japan) under conditions wherein the revolution rate of the silicon waferwas 1,500 rpm. The resultant coating was heated in the air at 120° C.for one hour and, then at 180° C. for one hour, thereby obtaining asilicon oxide-organic polymer composite thin film. (The above-mentionedtitanium nitride thin film was formed by the reactive sputtering using atitanium target and a gaseous mixture of argon and nitrogen (totalpressure=0.27 Pa, partial pressure ratio=50:50).) The obtained compositethin film was subjected to heat treatment at 400° C. in a nitrogenatmosphere for one hour to burn off only the organic polymer in thesilicon oxide-organic polymer composite thin film, thereby obtaining aporous silicon oxide thin film. The thickness of the obtained poroussilicon oxide thin film was 0.50 μm. Then, on the surface of the thusobtained porous silicon oxide thin film was vacuum-deposited aluminumthrough a mask, to thereby form, on the porous silicon oxide thin film,electrodes each having a diameter of 1.7 mm. Using the thus obtainedstructure, the dielectric constant of the porous silicon oxide thin filmat 1 MHz was measured. As a result, it was found that the dielectricconstant was 2.01.

Reference Example 2

1.2 g of tetraethoxysilane and 0.68 g of polyethylene glycolmonomethacrylate having a number average molecular weight of 360 weredissolved in a mixed solvent of 2.0 g of N-methylpyrrolidone and 1.0 gof propylene glycol methyl acetate. To the resultant solution were added0.75 g of water and 0.15 g of 0.1 N nitric acid, followed by stirring atroom temperature for 2 hours. The resultant solution was coated onto asilicon wafer, using a spin coater (trade name: 1H-360S Spin Coater,manufactured and sold by Mikasa Co., Ltd., Japan) under conditionswherein the revolution rate was 1,500 rpm. The resultant coating washeated in the air at 120° C. for one hour and, then at 180° C. for onehour, thereby obtaining a sample structure comprising the silicon waferhaving, formed thereon, a silicon oxide-organic polymer composite thinfilm having a thickness of 0.41 μm. The obtained sample structure wassubjected to heat treatment at 450° C. in a nitrogen atmosphere for onehour to thereby burn off only the organic polymer in the siliconoxide-oraganic polymer composite thin film, thereby obtaining a poroussilicon oxide thin film. The thickness of the obtained thin film was0.32 μm. That is, the thickness decrease caused by the removal of theorganic polymer (which is calculated by the formula: the thickness ofthe silicon oxide-organic polymer composite thin film−the thickness ofthe porous silicon oxide thin film)/the thickness of the siliconoxide-organic polymer composite thin film) was 22%.

Reference Example 3

Substantially the same procedure as in Reference Example 2 was repeated,except that a polyethylene glycol having a number average molecularweight of 20,000, which had no polymerizable functional group, was usedinstead of polyethylene glycol monomethacrylate, thereby obtaining asample structure comprising the silicon wafer having, formed thereon, asilicon oxide-organic polymer composite thin film having a thickness of1.451 μm. The obtained sample structure was subjected to heat treatmentat 450° C. in a nitrogen atmosphere for one hour to thereby burn offonly the organic polymer in the silicon oxide-organic polymer compositethin film, thereby obtaining a porous silicon oxide thin film. Thethickness of the obtained thin film was 1.07 μm. That is, the thicknessdecrease caused by the removal of the organic polymer (which iscalculated by the formula: (the thickness of the silicon oxideorganicpolymer composite thin film−the thickness of the porous silicon oxidethin film)/the thickness of the silicon oxide-organic polymer compositethin film)) was 26%.

Industrial Applicability

By the method of the present invention which is based on the damasceneprocess (this process is commercially advantageous not only in that alow resistance metal, such as copper or silver, can be used as amaterial for a circuit, but also in that this process is suitable forthe production of a multilayer circuit board), not only can theline-to-line capacitance in the circuit structure be lowered, but also alow resistance metal, such as copper or silver, can be used as amaterial for a circuit, so that it has become possible to produce anexcellent circuit structure in which the delay in the transmission ofthe electric signal (i.e., the interconnect delay) is greatlysuppressed, as compared to the case of the conventional circuitstructures. Further, by the method of the present invention, it hasbecome possible to produce effectively, efficiently such an excellentcircuit structure.

The multilayer circuit board and the semiconductor device, eachcomprising the above-mentioned excellent circuit structure produced bythe method of the present invention, exhibit excellent performance,since the interconnect delay is greatly suppressed.

What is claimed is:
 1. A method for producing a circuit structurecomprising: (a) a substrate, (b) an insulator layer formed on saidsubstrate, said insulator layer comprising a porous silicon oxide thinfilm having a groove, said groove defining a pattern for a circuit, and(c) a circuit comprising a metal layer formed in said groove, saidmethod comprising the steps of: (1) forming a preliminary insulatorlayer comprising a silicon oxide-organic polymer composite thin filmformed on said substrate, said silicon oxide-organic polymer compositethin film comprising a silicon oxide having an organic polymer dispersedtherein, (2) forming, in said preliminary insulator layer, a groovewhich defines a pattern for a circuit, (3) forming, in said groove, ametal layer which functions as a circuit, and (4) removing said organicpolymer from said silicon oxide-organic polymer composite thin film ofsaid preliminary insulator layer to render said preliminary insulatorlayer porous, thereby converting said preliminary insulator layer to aninsulator layer comprising a porous silicon oxide thin film.
 2. Themethod according to claim 1, wherein the removal of said organic polymerfrom said preliminary insulator layer is conducted by lightirradiation-heat treatment.
 3. The method according to claim 2, whereinsaid organic polymer in said silicon oxide-organic polymer compositethin film comprises at least one polymer selected from the groupconsisting of a polymer having a graft structure and a polymer having athree-dimensional network structure.
 4. The method according to claim 1,wherein said organic polymer in said silicon oxide-organic polymercomposite thin film comprises at least one polymer selected from thegroup consisting of a polymer having a graft structure and a polymerhaving a three-dimensional network structure.
 5. A circuit structureproduced by the method of claim
 1. 6. A multilayer circuit boardcomprising a laminate of a plurality of circuit structures, wherein atleast one circuit structure of said circuit structures is the circuitstructure of claim
 3. 7. A semiconductor device comprising the circuitstructure of claim 4.